I do. It may be minimal but making all the changes - no snorkel, UNI filter, and bellmouth, gets the most you can out of the airbox and is very noticeable. I wanted to keep the airbox and my dyno runs are better than people that have replaced theirs with way more expensive alternatives. My service dealer (D&D Cycles) believes airbox removal is not necessary to get the best performance out of the bike.
BTW, I bought the Norman Hyde bellmouth which is WAY more expensive than the Polaris. I assume the Polaris would have close results. No regrets on my purchase and it now looks like it was supposed to be that way. One other thing - you can actually hear the difference, so it must be doing something.
Norman Hyde bellmouth 
